301 old site to new with lots of url structure changes

I will soon be 301’ing my entire site to its new home on a new domain, where it will live as a subdirectory of the main domain. It lives at the root of its current domain. Now, if every single old page had the exact same url structure as every single new page, the redirects are trivial. However, of the 200ish old urls that will need to be redirected, very few will exist at their new site “equivalent” (www.olddomain.com/somepage -> www.newdomain.com/olddomain/somepage) as most pages will have either a slight url change or exist somewhere else entirely, like at the root of the new site.

I’m assuming 200ish lines of specific singular 301 redirects in .htaccess would slow down the site pretty bad since .htaccess is loaded for every asset on the page, correct? As would (relatively) complex conditional logic with mod_rewrite?

Any ideas?

Side note: both sites, new and old, use WordPress, so if there are any WordPress-specific solutions, fire away.